Get Key Value

Installation

npm install get-key-value

yarn add get-key-value

Purpose

If you have been using Typescript you have likely stumbled into the Element implicitly has an 'any' type because expression of type 'string' can't be used to index error.

This tiny package makes it easy to deal with this error.

Explanation of the Warning

const user = {name: 'Bill'}

const getKeyValueA = (key: string, obj: object) => obj[key]
getKeyValueA('name', user) // Won't work

const getKeyValueB = (key: string, obj: Record<string, any>) =>
  obj[key]
getKeyValue('name', user) // Will work but at the loss of type safety

const getKeyValue = <U extends keyof T, T extends object>(
  key: U,
  obj: T
) => obj[key]
getKeyValue<keyof typeof user, typeof user>('name', user) // Will work with type safety

getKeyValueA

• We set the key parameter to be type string
• We set the obj parameter to be type object
• The object type defaults to an empty object {}
• The are no string in an empty object, therefore a string cannot be used to retrieve a value

getKeyValueB

• We set the key parameter to be type string
• We set the obj parameter to be type Record<string, any>

// Valid Record<string, any>
const user = {
  name: 'Bill',
  age: 20,
  likes: ['turtles', 'rock climbing'],
}
// Invalid Record<string, any>
const user = {1: 'Bill', 2: 20, 3: ['turtles', 'rock climbing']}

• Whilst this works, we lose typesafety as any string can be used to index an object and the type of the value will be any

getKeyValue

• We set the key parameter to be the generic U which is a keyof T
• We set the obj parameter to be the generic T which extends object
• T extends an empty object and U extends the keys of T. Therefore U will always exist on T and can be used as a look up value.

Usage

There are two ways to use the getKeyValue function:

Option A

Use this option if you have a interface defined for your object

interface User {
  name: string
  age: number
}
const user: User = {name: 'John Smith', age: 20}
getKeyValue<keyof User, User>('name', user) // => 'John Smith'

Option B

Use this option if you don't have an interface defined for your object

const user = {name: 'John Smith', age: 20}
getKeyValue<keyof typeof user, user>('name', user) // => 'John Smith'